1. Field of the Invention
The present invention relates to a method of preparing N-formyl-L-aspartic anhydride as an intermediate for the production of a dipeptide-based sweetener .alpha.-L-aspartylphenylalanine methyl ester. More particularly, the present invention relates to a method of preparing N-formyl-.alpha.-L-aspartyl-L-phenylalanine and N-formyl-.alpha.-L-aspartyl-L-phenylalanine methyl ester using N-formyl-L-aspartic anhydride as a starting material.
2. Discussion of the Background
N-formyl-L-aspartic anhydride (hereinafter referred to as FAA) is usually obtained by the reaction of N-formyl-L-aspartic acid with formic acid and acetic anhydride (hereinafter referred to as dehydration reaction). As such a reaction known is a reaction in which acetic acid and formic acid are employed in almost stoichiometric amounts based on aspartic acid (U.S. Pat. No. 4,526,985 and U.S. Pat. No. 4,810,816), which is problematic in that the yield is low as based on the amount of aspartic acid as the starting material.
In order to increase the yield of the dehydration reaction, it is necessary to use formic acid and acetic anhydride both in excessive amounts relative to the amount of aspartic acid. In such a case, a significant amount of unreacted formic acid remains at the end of the reaction.
When .alpha.-L-aspartyl-L-phenylalanine methyl ester (hereinafter referred to as .alpha.-APM) is produced as a final product, FAA is reacted subsequently with L-phenylalanine or its methyl ester (hereinafter referred to as PM) to yield N-formyl-.alpha.-L-aspartyl-L-phenylalanine (hereinafter referred to as F-.alpha.-AP) or its methyl ester (hereinafter referred to as F-.alpha.-APM), (hereinafter this reaction is referred to as condensation reaction). In such a condensation reaction, formic acid, if remains, inhibits the reaction and serves to reduce the yield of N-formyl-.alpha.-L-aspartyl-L-phenylalanine or its methyl ester (.alpha. form) relative to N-formyl-.beta.-L-aspartyl-L-phenylalanine or its methyl ester (.beta. form). There is no effective means to convert such a .beta. form once produced to an .alpha. form in a subsequent step such as a deformylation step, and .beta.-APM derived from the .beta. form exhibits no sweetness. Accordingly, the reduction in the production ratio of the .alpha. form and the .beta. form (.alpha./.beta. ratio) in the condensation process results in an increase in production loss, which leads to an increase in the amount of the starting materials and their reactants.
In an attempt to remove formic acid, a method of evaporating the dehydration reaction fluid to dryness has been described (U.S. Pat. No. 3,933,781). However, in view of the stability of the reaction products and the handling at an industrial scale, preferably the anhydride suspended in the dehydration reaction fluid is once subjected to solid/liquid separation, and then the isolated anhydrous crystal is subjected to the condensation reaction. It is also known that the isolation yield can be increased by conducting this separation in the presence of an aromatic hydrocarbon and/or halogenated hydrocarbon (JPA S51-91210).
Although it is preferable to separate the FAA from the reaction fluid by centrifugation in order to reduce as much as possible the residual solvent in FAA crystals after the separation, the centrifugation process involves problems that are characteristic of this system. For example, one problem is due to the fact that FAA crystals produced by the method mentioned above are in a form of a column having a diameter and height both as large as from 1 mm to 3 mm, resulting in a great intrinsic specific gravity which leads to quite rapid sedimentation. Accordingly, when the suspension is supplied to the filter whose basket has already started rotation, the filtration is completed instantly and therefore the separated FAA crystals which are formed on the basket at a certain thickness, i.e., the cake, shows local deposition without a uniformity in thickness. Such a condition affects the safety of the operation of the device, and makes it difficult to operate it continuously. As an attempt to avoid such unbalanced cake deposition, proposed is a method of supplying the suspension while rotating the basket at a quite low rate. However, a too low rotation rate results in the deposition of the cake concentrated in the place under the basket. Thus, the reduction in the rotation rate is also limited, and a rotation rate smaller than such limit fails to completely prevent unbalanced deposition.
Once such unbalanced deposition of the cake is formed at the time of fluid supply, subsequent filtration and dehydration should be conducted also at a very low rotation rate in view of safety. Accordingly, the cake ultimately obtained contains a large amount of fluid, i.e., a large amount of residual formic acid.
Also when washing the cake with a washing fluid, the efficiency of the washing varies by location depending on the thickness of the cake which results in another problem. On average, such variation causes extremely low washing efficiency when compared with the washing of a uniformly deposited cake. Considering the influence of the residual formic acid on the yield of the condensation reaction, it is a matter of course that the problems mentioned above become significant in cases of the production of .alpha.-APM as a final product.
On the other hand, when comparing the affinities of FAA with various solvent components in the dehydration reaction fluid (formic acid, acetic acid, acetic anhydride, aromatic hydrocarbon), the affinity for formic acid is especially high. Accordingly, the concentration of formic acid in the laminar film of the surface of the FAA crystals is higher than those in other regions of the reaction fluid. Therefore, in view of reducing the residual formic acid, it is desirable that the fluid is stirred vigorously when conducting the centrifugation to obtain a laminar film as thin as possible prior to the filtration. However, when a conventional centrifugation method is used, only a gentle fluidization such as through pumping of the suspension is achieved and then the filtration is conducted rapidly without an interval for obtaining a thinner laminar film.
Because of the various problems mentioned above, it is impossible to employ centrifugation to isolate FAA, and the separation is typically conducted using a pressurized plate filtration machine. However, such procedure involves complicated operations and is not suitable for large-scale production. Accordingly, in large scale production, a dehydration reaction not requiring the separation of FAA, i.e., the reaction using formic acid in a stoichiometric amount based on aspartic acid, should be employed in a practical stage.